Electric power steering device

ABSTRACT

An electric motor ( 7 ) is driven with high output at the time of low-speed running, and a steering at a high resolution is enabled at the time of fast-speed running. 
     An electric power steering device ( 1 ) causes the electric motor ( 7 ) to generate auxiliary torque in accordance with the largeness of steering wheel torque by the steering operation of a wheel ( 9 ) made by a driver and reduces the steering wheel torque to the driver. A control device ( 10 ) drives, based on a vehicle-speed signal (Vs) from a speed sensor ( 11 ), the electric motor ( 7 ) with high output at the time of low-speed running, and enables steering at a high resolution at the time of fast-speed running without increasing an electric-motor output. Moreover, large steering wheel torque is requisite at the time of low-speed running, and a steering at a high resolution is requisite at the time of fast-speed running. Accordingly, this does not affect the steering feeling to the driver.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric power steering device whichassists steering wheel torque with a multiphase AC motor.

2. Description of the Related Arts

An electric power steering device causes an electric motor to generateauxiliary torque in accordance with steering wheel torque by a steeringoperation by a driver and reduces the steering wheel torque.

It is requisite for the electric power steering device to perform acontrol to the electric motor which does not affect the steering feelingto the driver.

PWM control by a triangular-wave comparison technique is for comparing athree-phase sine-wave command voltage with a reference triangular-wavevoltage and for generating a PWM-control signal voltage. A PWM invertergenerates, based on the PWM-control signal voltage, a rectangular-wavedrive voltage having undergone the PWM control, applies therectangular-wave drive voltage to the electric motor, and allows athree-phase current to flow.

Moreover, when a PWM control is performed using a PWM-control signalvoltage generated with the amplitude ratio of the three-phase sine-wavecommand voltage being increased, the amplitude ratio is limited to apredetermined amplitude ratio because the average voltage of a PWM drivevoltage fluctuates and the torque ripple of the electric motorincreases. The amplitude ratio is, however, the ratio of the amplitudeof a signal voltage relative to the amplitude of the referencetriangular-wave voltage.

Non-patent document 1 discloses a technology of increasing the rate ofutilization of voltage. According to such technology, a three-phasemodulation-wave voltage that is a three-phase sine-wave command voltagemodulated by a third-order integral-multiple harmonic (a triangularwave) is used for the PWM control.

The three-phase modulation-wave voltage becomes small in the vicinity ofthe amplitude of the signal voltage in comparison with the three-phasesine-wave command voltage. As a result, the average voltage of the PWMdrive voltage does not fluctuate, and a range where the amplitude ratiocan be set large expands, thereby improving the rate of utilization ofvoltage.

Non-Patent document 1: Theory of AC servo system and designing thereofin practice, chapter three, power converter circuit pages 44 to 45 (Jul.10, 2005, seventh edition, published by: SOUGOU DENSHI SHUPPAN)

The technology disclosed in Non-patent document 1 improves the rate ofutilization of voltage and increases a current flowing through theelectric motor (multiphase AC motor), so that the electric motor can bedriven with high output.

An electric power steering device to which the technology disclosed inNon-patent document 1 is applied does not affect the steering feeling toa driver even if the resolution of steering becomes small at the time oflow-speed running (at the time of static steering).

However, as it is requisite to perform steering at a high resolution atthe time of fast-speed running, the electric power steering device towhich the technology disclosed in Non-patent document 1 is appliedaffects the steering feeling to the driver at the time of fast-speedrunning.

SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoing problem,and it is an object of the present invention to provide an electricpower steering device which can drive a multiphase AC motor with highoutput in accordance with a vehicle speed to reduce steering wheeltorque, or which enables steering of a wheel at a high resolutionwithout reducing the steering wheel torque.

In order to overcome the foregoing problem, an electric power steeringdevice of the present invention according to claim 1 causes a vehicle tobe steered by torque generated by a multiphase AC motor driven inaccordance with a steering input, and comprises a control device whichadds a fundamental of a drive voltage applied to the multiphase AC motorto a harmonic component of the fundamental and which drives themultiphase AC motor based on a modulation wave acquired by addition,wherein the control device causes the harmonic component to be variablein accordance with a vehicle speed.

According to such configuration, the modulation wave (e.g., athree-phase modulation-wave voltage) which is attenuated in the vicinityof the maximum amplitude and planarized by adding the fundamental (e.g.,a three-phase sine-wave command voltage) to the harmonic component isapplied to the multiphase AC motor, thereby making an electric motoroutput increased.

In this case, as the harmonic component is caused to be variable inaccordance with the vehicle speed and an output by the multiphase ACmotor is also caused to be variable, the electric motor output can beincreased at the time of low-speed running and a steering at a highresolution is enabled at the time of fast-speed running.

Moreover, in the foregoing electric power steering device, the controldevice comprises: sine-wave generating means for generating a sine wavedependent on a rotation angle of the multiphase AC motor; a harmonicgenerator which generates a harmonic component with the sine wave beingas the fundamental; a variable controller which changes an amplitude ofthe harmonic component in accordance with the vehicle speed; amultiphase adder which adds the sine wave to the harmonic component togenerate the modulation wave; and a PWM inverter which performs PWMcontrol on the multiphase AC motor using the modulation wave.

According to the foregoing configuration, the multiphase AC motor isdriven by a PWM control with a small torque ripple by using athree-phase modulation wave acquired by adding a three-phase sine waveto a harmonic component having an amplitude which changes in accordancewith the vehicle speed.

Accordingly, at the time of low-speed running which requires largesteering wheel torque, the rate of utilization of voltage can beincreased and a voltage applicable to the multiphase AC motor can bealso increased, so that an electric motor output can be made increased.Moreover, at the time of fast-speed running which requires a steering ata high resolution, the steering is enabled at a high resolution withoutmaking the electric motor output increased. Consequently, this does notaffect the steering feeling to a driver.

Moreover, in a preferred embodiment, according to the electric powersteering device of the present invention as set forth in claim 3, thevariable controller is a switch which outputs a value acquired bysetting the amplitude of the harmonic component to zero when the vehiclespeed exceeds a predetermined value (e.g., a speed threshold Vs1 [km/h])and directly outputs a value of the harmonic component when the vehiclespeed is equal to the predetermined value or slower.

Furthermore, in a preferred embodiment, according to the electric powersteering device of the present invention as set forth in claim 4, theharmonic component is a triangular wave which is a harmonic odd-numbermultiple of the fundamental.

The triangular wave is the odd-number multiple harmonic, and can beeasily generated. This facilitates the designing of a harmonic generatorand the manufacturing thereof.

Moreover, the harmonic component includes a third-orderintegral-multiple harmonic relative to the fundamental.

In this case, the average voltage of a voltage (a line voltage) appliedto the multiphase AC motor contains no third-order integral-multipleharmonic component, so that a waveform does not become fluctuated.

According to the present invention, it becomes possible to drive amultiphase AC motor with high output in accordance with a vehicle speedto reduce steering wheel torque, or to enable steering of a wheel at ahigh resolution without reducing the steering wheel torque.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an electric power steering device accordingto an embodiment of the present invention;

FIG. 2 is a block diagram showing a control device and an electric motorboth in FIG. 1;

FIG. 3 is a diagram showing an operation of a switch in FIG. 2;

FIG. 4 is a diagram showing a signal voltage which is input/output by athree-phase adder in FIG. 2;

FIG. 5 is a diagram showing an operation performed by a PWM inverter inFIG. 2; and

FIG. 6 is a diagram showing a relationship between a phase voltage of aPWM drive voltage and a line voltage thereof.

DESCRIPTION OF REFERENCE NUMERALS

1 Electric power steering device

2 Steering wheel

3 Steering shaft

4 Pinion shaft

4A Pinion

5 Torque sensor

6 Torque transmitting means

7 Electric motor (multiphase AC motor)

8 Rack shaft

8A Rack tooth

9 Wheel

10 Control device

11 Speed sensor

15 High-voltage battery

20 PWM inverter

21 Angle sensor

22 Main body

25 Current sensor

30 Biaxial/three-phase coordinate converter (sine-wave generating means)

31 Harmonic generator

32 Three-phase adder (multiphase adder)

33 Switch (variable controller)

35 Three-phase/biaxial coordinate converter

50 Target-current converter

PREFERRED EMBODIMENT OF THE INVENTION First Embodiment

FIG. 1 is a diagram showing an electric power steering device 1according to an embodiment of the present invention. The electric powersteering device 1 comprises a steering wheel 2, a steering shaft 3, apinion shaft 4, a pinion 4A, a torque sensor 5, torque transmittingmeans 6, an electric motor 7, a rack shaft 8, a rack tooth 8A, twowheels 9, a control device 10, and a speed sensor 11. An example of theelectric motor 7 which is a multiphase AC motor is a three-phasebrushless motor.

A driver steers the running direction of a vehicle through the electricpower steering device 1 by manipulating the steering wheel 2.

The steering wheel 2 transmits rotary force based on steering wheeltorque from the driver to the torque sensor 5, to the torquetransmitting means 6, and to the pinion 4A through the steering shaft 3and the pinion shaft 4.

The pinion 4A and the rack tooth 8A mesh together to convert the rotaryforce into force for a linear motion in the axial direction of the rackshaft 8. The force from the rack shaft 8 for a linear motion acts on thetwo wheels 9, 9 which change a direction by what corresponds to asteering angle in accordance with the rotary force. As a result, therunning direction of the vehicle changes in accordance with themanipulation given by the driver.

The torque sensor 5 detects steering wheel torque applied to thesteering shaft 3 in accordance with a steering operation by the driverthrough the steering wheel 2, generates an electrical torque signal T,and outputs the torque signal T to the control device 10. The speedsensor 11 detects the velocity of the vehicle (a vehicle speed) andoutputs a vehicle-speed signal Vs to the control device 10.

The electric motor 7 generates auxiliary torque based on three-phasecurrents Iu, Iv, and Iw, and transmits the generated auxiliary torque tothe pinion 4A and to the rack shaft 8 through the torque transmittingmeans 6. As a result, the steering wheel torque of the driver isreduced. The control device 10 generates a PWM drive voltage which is arectangular-wave voltage based on the torque signal T, on thevehicle-speed signal Vs, and on an angular signal θ, applies thegenerated PWM drive voltage to the electric motor 7, and allows thethree-phase currents Iu, Iv, and Iw to flow.

Moreover, the electric motor 7 outputs the angular signal θ whichrepresents the rotation angle of the electric motor 7 to the controldevice 10.

FIG. 2 is a block diagram showing the control device 10 and the electricmotor 7 both in FIG. 1. The control device 10 has a target-currentconverter 50, adders 70, 75, a current controller 80, abiaxial/three-phase coordinate converter 30, a three-phase adder 32, aharmonic generator 31, a switch 33 (a variable controller), and athree-phase/biaxial coordinate converter 35, and respective functions ofthose units are realized by a computer having a CPU, a ROM and a RAM,and by a program.

Furthermore, the control device 10 has a PWM inverter 20, a high-voltagebattery 15, and a current sensor 25.

The electric motor 7 has a main body 22 and an angle sensor 21, and themain body 22 has a stator and a rotator, and the stator has at leastthree stator coils. The three stator coils each has one end connected toa neutral point and the other end connected to the terminal of the mainbody 22, and are connected in a star connection manner. The rotator ofthe main body 22 has a rotary shaft which is rotatably supported, androtary force is applied thereto by a magnetic field generated by thethree stator coils.

The control device 10 performs dq vector control which decomposes acurrent into a magnetic-pole axis component and into a torque axiscomponent, and also performs feedback control so as to cause a deviationbetween a q-axis current command value iq* and a q-axis electric-motorcurrent value iq to be zero. A d-axis component is the magnetic-poleaxis component, and a q-axis component is the torque axis component. Thecurrent sensor 25 detects respective current values of the two phasecurrents Iu, and Iv among the three-phase currents of the electric motor7, and transmits a U-phase electric-motor current value in and a V-phaseelectric-motor current value iv to the three-phase/biaxial coordinateconverter 35. The angle sensor 21 detects the rotation angle of therotary shaft of the electric motor 7 and outputs the angular signal θ tothe biaxial/three-phase coordinate converter 30, to the harmonicgenerator 31, to the PWM inverter 20, and to the three-phase/biaxialcoordinate converter 35.

The three-phase/biaxial coordinate converter 35 performsthree-phase/biaxial coordinate conversion based on the U-phaseelectric-motor current value iu, on the V-phase electric-motor currentvalue iv, and on the angular signal θ, and generates a d-axiselectric-motor current value id and the q-axis electric-motor currentvalue iq. Moreover, a W-phase electric-motor current value iv isacquired through a calculation based on an equation: iu+iv+iw=0. Thecalculation of the three-phase/biaxial coordinate conversion isperformed by using, for example, a following equation.

$\begin{matrix}{\begin{bmatrix}{Id} \\{Iq}\end{bmatrix} = {{\sqrt{2}\begin{bmatrix}{\sin \left( {\theta + {\pi/3}} \right)} & {\sin \; \theta} \\{\cos \left( {\theta + {\pi/3}} \right)} & {\cos \; \theta}\end{bmatrix}}\begin{bmatrix}{Iu} \\{Iv}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

where θ in the equation is an electric angle, and is a value acquired bymultiplying the mechanical rotation angle (mechanical angle) of therotator by the number of polar pairs.

The three-phase/biaxial coordinate converter 35 outputs the d-axiselectric-motor current value id to the adder 75, and outputs the q-axiselectric-motor current value iq to the adder 70.

The target-current converter 50 generates, based on the torque signal Tand on the vehicle-speed signal Vs, the corresponding q-axis currentcommand value iq* and outputs the generated value to the adder 70. Notethat when a weakened field control is not performed, a d-axis currentcommand value id* is set to zero.

The adder 70 subtracts the q-axis electric motor-current value id fromthe q-axis current command value iq* and outputs the subtraction resultto the current controller 80. The adder 75 subtracts the d-axiselectric-motor current value id from the q-axis current command valueid* and outputs the subtraction result to the current controller 80.

The current controller 80 performs proportional/integral control (PTcontrol) on respective output signals by the adders 70, 75, generatescorresponding q-axis voltage command value Vq* and d-axis voltagecommand value Vd*, and outputs those generated values to thebiaxial/three-phase coordinate converter 30.

The biaxial/three-phase coordinate converter 30 is sine-wave generatingmeans which generates a sine wave dependent on the rotation angle of theelectric motor 7, performs biaxial/three-phase coordinate conversionbased on the q-axis voltage command value Vq*, on the d-axis voltagecommand value Vd*, and on the angular signal θ, and generatesthree-phase sine-wave command voltages Vu*, Vv*, and Vw*. Thecalculation of the biaxial/three-phase coordinate conversion isperformed by using, for example, a following equation.

$\begin{matrix}{\begin{bmatrix}{{Vu}*} \\{{Vv}*} \\{{Vw}*}\end{bmatrix} = {{\sqrt{2/3}\begin{bmatrix}{\cos \; \theta} & {{- \sin}\; \theta} \\{\cos \left( {\theta - {{2/3}\pi}} \right)} & {- {\sin \left( {\theta - {{2/3}\pi}} \right)}} \\{\cos \left( {\theta + {{2/3}\pi}} \right)} & {- {\sin \left( {\theta + {{2/3}\pi}} \right)}}\end{bmatrix}}\begin{bmatrix}{{Vd}*} \\{{Vq}*}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

The biaxial/three-phase coordinate converter 30 outputs the three-phasesine-wave command voltages Vu*, Vv*, and Vw* to the three-phase adder32. The three-phase sine-wave command voltages Vu*, Vv*, and Vw* arethree sine-wave signal voltages each having a phase difference of 120degree one another, and having the same frequency and amplitude.

The harmonic generator 31 generates, based on the angular signal θ, asynchronous triangular-wave voltage Vm which is a harmonic component ofthe three-phase sine-wave command voltages Vu*, Vv*, and Vw*, andoutputs the generated voltage to the switch 33. The synchronoustriangular-wave voltage Vm is, for example, a triangular wave, and is asignal voltage generated as the synchronized odd-number multiple ofharmonic component is overlapped relative to the three-phase sine-wavecommand voltages Vu*, Vv*, and Vw*.

FIG. 3 is a diagram showing an operation of the switch 33 in FIG. 2. Theswitch 33 is a variable controller, does not change the amplitude of thesynchronous triangular-wave voltage Vm when the vehicle-speed signal Vsis equal to a speed threshold Vs1 [km/h] or slower, and sets theamplitude value of the synchronous triangular-wave voltage Vm to zerowhen the vehicle-speed signal Vs exceeds the speed threshold Vs1. Theswitch 33 outputs the synchronous triangular-wave voltage Vm to thethree-phase adder 32.

Note that a value indicating a boundary between when the vehicle runs ata slow speed and when the vehicle runs at a fast speed is set as thespeed threshold Vs1.

FIG. 4 is a diagram showing a signal voltage which is input/output bythe three-phase adder 32 in FIG. 2. The vertical axis represents avoltage [V], and the horizontal axis represents an angle [degree]. Thethree-phase adder 32 (a multiphase adder) subtracts the amplitude valueof the synchronous triangular-wave voltage Vm from each amplitude valueof the three-phase sine-wave command voltage Vu*, Vv*, and Vw*,generates three-phase modulation-wave voltages Vu, Vv, and Vw,respectively, and outputs those generated voltages to the PWM inverter20 (see FIG. 2).

Moreover, this figure shows a case in which the switch 33 (see FIG. 2)does not change the amplitude of the synchronous triangular-wave voltageVm. The three-phase modulation-wave voltages Vu, Vv, and Vw attenuate inthe vicinity of respective maximum amplitudes in comparison with thethree-phase sine-wave command voltages Vu*, Vv*, and Vw*. Accordingly,the amplitudes become small by 2Δ A as a whole.

Note that when the switch 33 sets the amplitude of the synchronoustriangular-wave voltage Vm to zero, respective waveforms of thethree-phase modulation-wave voltages Vu, Vv, and Vw become the same sinewaveforms as waveforms of the three-phase sine-wave command voltagesVu*, Vv*, and Vw*, respectively. Moreover, the three-phase adder 32 maybe configured so as to multiply by a predetermined gain in accordancewith a vehicle speed instead of the switch.

FIG. 5 is a diagram showing an operation performed by the PWM inverter20 in FIG. 2. To facilitate explanation, the respective waveforms of thethree-phase modulation-wave voltages Vu, Vv, and Vw are waveforms in acase in which the switch 33 sets the amplitude of the synchronoustriangular-wave voltage Vm to zero (the vehicle-speed signal Vs exceedsthe speed threshold Vs1 [km/h]).

FIG. 5( a) is a diagram showing a PWM conversion performed by the PWMinverter 20. The PWM inverter 20 compares a reference triangular-wavevoltage Vc generated by a non-illustrated triangular-wave generator witheach of the three-phase modulation-wave voltages Vu, Vv, and Vw,performs PWM conversion by a triangular-wave comparison technique, andgenerates a PWM-control signal voltage. The reference triangular-wavevoltage Vc has a frequency set to be higher than those of thethree-phase modulation-wave voltages Vu, Vv, and Vw. As the frequency ofthe reference triangular-wave voltage Vc is set to be high, a torqueripple generated by the electric motor 7 becomes small.

As shown in FIG. 2, the high-voltage battery 15 supplies a DC voltage Eato the PWM inverter 20. The PWM inverter 20 turns on/off non-illustratedplural switching elements at predetermined timings based on thePWM-control signal voltage.

FIG. 5( b) is a diagram showing a phase voltage of a PWM drive voltageapplied to the electric motor 7 in FIG. 2. Phase voltages Vun, Vvn, andVwn each has an amplitude that is the DC voltage Ea, and arerespectively applied between the U-phase terminal of the main body 22 ofthe electric motor 7 and the neutral point, between the V-phase terminalof the main body 22 and the neutral point, and between the W-phaseterminal of the main body 22 and the neutral point.

FIG. 5( c) is a diagram showing a line voltage of the PWM drive voltageapplied to the electric motor 7 in FIG. 2. Line voltages Vuv, Vvw, andVwu each has an amplitude twice as much as the DC voltage E2, and arerespectively applied between the U-phase terminal of the main body 22 ofthe electric motor 7 and the V-phase terminal thereof, between theV-phase terminal of the main body 22 and the W-phase terminal thereof,and between the W-phase terminal of the main body 22 and the U-phaseterminal thereof.

FIG. 6 is a diagram showing a relationship between a phase voltage ofthe PWM drive voltage and a line voltage thereof. The electric motor 7generates auxiliary torque in accordance with steering wheel torque asthe three-phase currents Iu, Iv, and Iw in accordance with the linevoltages Vuv, Vvw, and Vwu flow through the electric motor 7.

In FIG. 5( c), each of the line voltages Vuv, Vvw, and Vwu which areapplied to the electric motor 7 has a pulse width wide in the vicinityof a center part and has a pulse width narrow in the vicinity of bothend parts in each time per a half period. Accordingly, an averagevoltage in one period becomes an equivalent sine-wave voltage.

In comparison with a case in which merely a voltage having a pulse widthconstant is applied to the electric motor 7, as the line voltages Vuv,Vvw, and Vwu having undergone the PWM control are applied, the averagevoltage of the PWM drive voltage does not fluctuate and the harmoniccomponent is little, so that a torque ripple generated by the electricmotor 7 becomes small.

Together with the increase of the torque signal T, respective amplitudesof the three-phase sine-wave command voltages Vu*, Vv*, and Vw* increaseso that the electric motor 7 generates corresponding auxiliary torque.Likewise, respective amplitudes of the three-phase modulation-wavevoltages Vu, Vv, and Vw increase.

However, when the respective amplitudes of the three-phasemodulation-wave voltages Vu, Vv, and Vw exceed the amplitude of thereference triangular-wave voltage Vc, the average voltage of the PWMdrive voltage (the line voltage) fluctuates and the harmonic componentincreases, causing the electric motor 7 to generate a torque ripple.Accordingly, respective maximum amplitude ratios of the three-phasemodulation-wave voltages Vu, Vv, and Vw are limited to respectivepredetermined values (see, page 44 of Non-patent document 1).

When the switch 33 sets the amplitude of the synchronous triangular-wavevoltage Vm to zero, although a maximum amplitude ratio that does notcause the average voltage of the PWM drive voltage (the line voltage) tofluctuate is set, the amplitude value of the fundamental component ofthe PWM drive voltage (the line voltage) becomes smaller than the DCvoltage Ea. Accordingly, the rate of utilization of voltage is low.Here, the rate of utilization of voltage is the ratio of the amplitudevalue of the fundamental component of the PWM drive voltage (the linevoltage) relative to the DC voltage Ea.

Conversely, when the switch 33 does not change the amplitude of thesynchronous triangular-wave voltage Vm, a maximum amplitude ratio thatdoes not cause the average voltage of the PWM drive voltage (the linevoltage) to fluctuate is large in comparison with a case in which theamplitude of the synchronous triangular-wave voltage Vm is set to zero.That is, as shown in FIG. 4, the three-phase modulation-wave voltagesVu, Vv, and Vw can have respective amplitude values increased by voltage2Δ A.

Accordingly, respective amplitudes of the fundamental components of theline voltages Vuv, Vvw, and Vwu based on the three-phase modulation-wavevoltages Vu, Vv, and Vw each having the amplitude value increased by thevoltage 2Δ A can be also increased, so that the rate of utilization ofvoltage is also improved.

Moreover, when the amplitude of the synchronous triangular-wave voltageVm is not changed, the maximum rate of utilization of voltage that doesnot cause the average voltage of the PWM drive voltage (the linevoltage) to fluctuate increases in comparison with a case in which theamplitude of the synchronous triangular-wave voltage Vm is set to zero,and the amplitude value of the fundamental component of the PWM drivevoltage (the line voltage) becomes substantially equal to the DC voltageEa. Accordingly, as a maximum voltage value which can be applied to theelectric motor 7 can be increased, respective maximum current values ofthe three-phase currents Iu, Iv, and Iw flowing in accordance with themaximum voltage value increase, thereby increasing maximum torquegenerated by the electric motor 7.

As a result, respective current values of the three-phase currents Iu,Iv, and Iw necessary for acquiring the same electric-motor output can bereduced. In this case, any loss inherent to resistive components likethe stator coils is reduced, and the power efficiency is improved.

The control device 10 determines whether or not to set the amplitudevalue of the synchronous triangular-wave voltage Vm to zero inaccordance with the vehicle-speed signal Vs.

The electric power steering device 1 generates large steering wheeltorque at the time of low-speed running (at the time of staticsteering), and enables steering at a high resolution without increasingsteering wheel torque at the time of fast-speed running. Moreover, largesteering wheel torque is requisite at the time of low-speed running, anda steering at a high resolution is requisite at the time of fast-speedrunning. Accordingly, the torque ripple by the electric motor 7 issmall, which does not affect the steering feeling to the driver.

Second Embodiment

In the control device 10 (see, FIG. 2), the switch 33 which selectswhether or not to set the amplitude value of the synchronoustriangular-wave voltage Vm to zero in accordance with the vehicle-speedsignal Vs is exemplified as the “variable controller” of the firstembodiment.

In a second embodiment, the control device 10 comprises a variablecontroller which causes the amplitude of a synchronous triangular-wavevoltage Vm to be variable instead of the switch 33. In such case, thevariable controller successively changes the amplitude of thesynchronous triangular-wave voltage Vm in accordance with avehicle-speed signal Vs.

Accordingly, it is also possible to increase the amplitude of thesynchronous triangular-wave voltage Vm along with the increase of thevehicle-speed signal Vs, so that the steering feeling to a driver can besuccessively changed in accordance with the vehicle-speed signal Vs.

First Modified Embodiment

The harmonic generator 31 may also generate, based on the angular signal0, the signal voltage of the synchronized third-order integral-multipleharmonic component with respect to the three-phase sine-wave commandvoltages Vu*, Vv*, and Vw*.

In this case, as the average voltage of the PWM drive voltage (the linevoltage) applied to the electric motor 7 contains no third-orderintegral-multiple harmonic component, the waveform does not becomefluctuated.

Second Modified Embodiment

Moreover, the control device 10 may also have a power amplifier insteadof the PWM inverter 20. In such case, the power amplifier performs poweramplification on the three-phase modulation-wave voltages Vu, Vv, and Vwand applies such amplified voltages to the electric motor 7. Theelectric motor 7 generates auxiliary torque based on the amplifiedthree-phase modulation-wave voltages Vu, Vv, and Vw. Accordingly, it ispossible to change an electric-motor output in accordance with thevehicle-speed signal Vs.

Third Modified Embodiment

Furthermore, the electric power steering device 1 of the presentinvention may include a steer-by-wire (Steer_By_Wire) that mechanicallyseparates the steering wheel 2 and the wheel 9 from each other.

1. An electric power steering device (1) which causes a vehicle to besteered by torque generated by a multiphase AC motor (7) driven inaccordance with a steering input, the electric power steering device (1)comprising: a control device (10) which adds a fundamental of a drivevoltage applied to the multiphase AC motor (7) to a harmonic componentof the fundamental and which drives the multiphase AC motor (7) based ona modulation wave acquired by addition, wherein the control device (10)causes the harmonic component to be variable in accordance with avehicle speed.
 2. The electric power steering device (1) according toclaim 1, wherein the control device (10) comprises: sine-wave generatingmeans (30) for generating a sine wave dependent on a rotation angle ofthe multiphase AC motor (7); a harmonic generator (31) which generates aharmonic component with the sine wave being as the fundamental; avariable controller (33) which changes an amplitude of the harmoniccomponent in accordance with the vehicle speed; a multiphase adder (32)which adds the sine wave to the harmonic component to generate themodulation wave; and a PWM inverter (20) which performs PWM control onthe multiphase AC motor (7) using the modulation wave.
 3. The electricpower steering device (1) according to claim 2, wherein the variablecontroller (33) is a switch (33) which outputs a value acquired bysetting the amplitude of the harmonic component to zero when the vehiclespeed exceeds a predetermined value, and directly outputs a value of theharmonic component when the vehicle speed is equal to the predeterminedvalue or slower.
 4. The electric power steering device (1) according toany one of claims 1 to 3, wherein the harmonic component is a triangularwave which is a harmonic odd-number multiple of the fundamental.
 5. Theelectric power steering device (1) according to any one of claims 1 to3, wherein the harmonic component includes a third-orderintegral-multiple harmonic relative to the fundamental.